Method for purification and bleaching of cellulosic materials



Patented Apr. 26,, 1932 UNITED STATES" PATENT orrlce CHARLES A.BLODGETT, OF SOUTH BREWER, MAINE, ASSIGNOR TO EASTERN MANU- FACTURINGCOMPANY, OF SOUTH BREWER, MAINE, A CORPORATION OF MASSAe CHU SETTSMETHOD FDR PURIFICATION AND BLEAGHING F CELLULOSIG MATERIALS N0 Drawing.

This invention relates-to a method for the purification and bleaching ofcellulosic pulps and the like, which is more or less generallyapplicable, but especially adaptable to those pulps which have beenproduced by an initial reduction with acid sulfite liquors.

In the art of reducing cellulosic raw materials to the condition of apulp fromwhich paper or other products are to be made, the softening anddisintegrating solutions usually employed may be qualitatively dividedinto two classes, namely those which are essentially acid, such as thebisulfites of the alkalis and alkaline earths which are commonly used inthe sulfite processes, and those which are alkaline, such as causticsoda, sodium sulfide, etc., which are used in the soda and sulfateprocesses, respectively.

A fundamental diflerence between the 2 processes of pulp reduction asthus classified, lies in the differences in the properties of the acidand alkaline solutions used and in'the corresponding several effects ofthese classes of reagents upon the raw fibrous materials which aretreated therewith. These are in general well recognized, but it is hereto be especially pointed out and emphasized that, whereas an initialtreatment of raw stocks with strongly caustic or alkaline solutions notonly liberates and dissolves the various components of the wood whichare associated with the fibres'but also tends to react with anddegenerate the alpha-cellulose constituent of the fibres themselves, theacid or bisulfite solution, on the other hand and by way of contrastwith the caustic treatments, are less reactive with respect to thealpha-cellulose component of the fibres, although potent reagents forliberating and removing most of the associated substances which arepresent in the raw wood.

Practical application of these observations has been made in the processdescribed in oopending application of Theodore Clark. SerialNo. 1,078,filed January 7, 1925, for

the production'of a pure alpha-cellulose pulp. The procedure theredisclosed typically includes a preliminary reduction of wood to pulp,treating the resulting pulp with a caustic soda solution having analkali concentra Application med February 16, 1926. Serial No. 88,684.

tion of the order of 10 to 15% sodium hydroxide or its equivalent, thenwashing and bleaching as by the addition of a mineral acid which issubsequently removed by washing. The finished pulp is dried or used inthe customary manner for the purpose desired.

It has also been found (co-pending application of Charles A. .Blodgettand Hugo H. Hanson Serial No. 87,806, filed February 12, 1926) that aprocedure somewhat analogous to that above outlined may be effectedwherein after a sulfite cook, preferably leaving the pulp somewhat raw,a second [or alkaline] stage of operations is conducted by digesting thepulp with a solution characterized by containing a reactive sulfurcompound in alkaline solution, preferably a reducin solution, such assodium sulfite, [thi-s1ilfate,] alkali sulfides, etc. or the like or amixture of these. In the aforesaid co-pending application this isfollowed by washlng the pulp and bleaching in the customary manner. But

the bleaching of pulp prepared by the process thus outlined may beespecially efiectively carried out by the procedure herein to bedescribed.

Another pulp-making process producing pulp particularly adapted to thebleaching process herein disclosed, is that of Vance P. Edwardes, U. S.Patent No. 1,310,694.

It has now been found, as a part of the bleaching agent added, oralternatively by checking short of completion by anti-chlor. Thisoxidizing'bleach is then followed by a treatment with a reducing agentsuch as an alkali or alkaline-earth bi-sulfite bleach, and finally bywashing of the pulp' with Water preferably until free from reagents. Theprocess is especially appropriate to those pulps which have beenpreviously cooked, in

any well known manner, with reducing acid liquors, such as theacid-sulfites, subsequently washed and then subjected to treatment withan alkaline reagent solution, for the further and more complete removalof initially associated lignin and other non-cellulose components, andof degenerated cellulosic components, of the pulp.

The process of the invention includes or presupposes apreliminaryreduction of appropriate cellulose-bearing materials such as Woods, moreespecially exemplified by those from the coniferous species of trees, tothe condition of a uniform fibrous pulp by partial digestion with areducing acid solution, for example one or more of the alkaline earthbi-sulfites, and further digestion in an alkaline reagent, followed bysubjecting to an initial reducing bleach, typically a solutionessentially containing sulphurous acid or a bi-sulfite (which alsoneutralizes any residual amounts of the alkaline reagent or solutionwhich the pulp may retain) treating with an oxidizing bleach such asbleaching powder, exhausting (or checking) the oxidizing bleachingreaction short of completion (preferably appreciably short of thecomplete or maximum bleaching effect obtainable by oxidizing action)whereafter a second and characteristically reducing bleach is effectedby the addition of a reducing bleaching reagent, typically a solutioncontaining sulfurous acid or a bi-sulfite, the excess of which isfinally separated and removed from the residual purified and highlybleached alphacellulose pulp, as by washing.

It is to be understood that certain other operations may be interspersedwith the several stages of procedure, as here outlined, for theprocurement of specific or improved results, to effect economies oftime, material, labor, etc., and also as incidental requirements due toany specific raw material employed and the variations of quality andcomposition which it manifests with respect to the several treatmentsinvolved.

Atypical and preferred application of the invention will be described ascarried out upon a pulp which has been prepared by the sulfite process,e. g. from woods of the species 1 of conifers, such as spruce, etc.,andwhich is further treated after the manner of the disclosures abovereferred to, for the procurement of a highly bleached alpha-cellulose ofhigh purity.

The wood is first reduced mechanically to the form of chips which arethen mixed with a customary solution of alkaline earthbisulfite,-usually containing both calcium and magnesium bi-sulfites andfree sulfurous acid, and digested under the conditions of heat andpressure, in the manner well known to those skilled in the art ofproducing sulfite pulp as by Mitscherlich or quick cook (Ritter-Kellner)processes. These conditions are maintained until sufficient action ishad upon the woodchips to soften and reduce them to the condition of auniform fibrous pulp. The various species of wood are adapted todifferent cooking methods, and hence a given batch should consist ofwoods adapted to the sulfite cook, and of a single species or of close-1y related species only, and not of mixed dissimilar woodsindiscriminately. Coniferous woods such as spruce, fir and hemlock forexample are well known to be adapted to the sulfite cook, while woods ofthe broadleaf or deciduous group such as poplar are not well adapted toit on account of their inherent characteristics and the southern pinesare not available without the preliminary extraction of pitch.

1n the instant case, where coniferous wood such as spruce is undertreatment, the digestion may be continued an appreciably less period oftime than that required for the usual complete sulfite digestion so thatonly a somewhat incomplete dissolution and removal of the lignin andother materials associated with the cellulose therein is effected, sothat the resulting pulp is of the character known as raw.

At this stage the alpha-cellulose constituent is left not onlysubstantially completely unchanged chemically but essentially withoutmaterial physical alteration or deterioration of its fibrous structure.On the other hand, such treatment penetrates the interstices between thefibres, dissolving most of the lignin and other non-celluloseconstituents normally present therein, and isolating the cellulosefibres but without causing the formation of appreciable quantities ofdegenerated celluloses or otherwise chemically affecting the principalor alpha-cellulose constituent of the wood.

The pulp so produced is next washed preferably to approximate neutralityand may then be subjected directly to the oxidizing bleach, although ifintended to be so bleached (without the alkaline secondary cook hereinreferred to) it will ordinarily be more completely cooked to produce aneasy bleaching grade.

For the manufacture of a higher grade of alpha-cellulose pulp, it isdesirable at this point to subject the pulp first to an alkalinetreatment in accordance with the procedures above alluded to, e. g. witha solution of sodium hydroxide or its equivalent, or with a relativelydilute solution (e. g. 3%. more or less) of a reactive sulfur compoundcharacterized by being alkaline,either per se or through the addition ofan alkali,such as a mixture of sodium sulfide and sodium bi-sullite. Inthe latter instance it is desirable to accompany the alkaline treatmentwith an increase of temperature and pressure. The batch so treated isthen separated and washed substantially free from alkaline liquor.

The improved bleaching procedure in preferred form is next described asapplied to such a pulp. 1

The residual traces of alkaline reagent which remain associated with thecharge are neutralized or rendered slightly acidic by the addition of areducing acid reagent such as a soluble bi-sulfite or sulfurousacid,conveniently freshly prepared cooking liquor of the sulfiteprocess, effecting a preliminary bleaching of the pulp which has beensomewhat darkened by the alkaline secondary cook. It is then ready forthe oxidizing bleach. The oxidizing bleach may be effected in the usualmanner, but with a bleach liquor containing available chlorine in totalamounts of the order of only 2 to 2 5% of v the weight of dry fibreinstead of the usual much larger amount. Oxidizing bleaching agents notonly react upon the coloring components of the pulp and the portionswhich it is desirablel to oxidize, render soluble, and remove, but alsoif 1n sufficient amount and concentration, tend to act in course of timeupon the cellulosic constituentsincluding' th'e alpha-cellulose,producing oxy-celluloses, etc., (and also imparting an additionalcoloration thereto). Moreover, overbleaching with such reagents alsolowers the physical strength of the fibres to an appreciable extent andreduces the desirable properties of the pulp generally.

It is now recommended, for the purposes of the present invention and toavoid such degradation of the pulp, to exhaust or check the bleachingaction considerably short of its ultimate possible effects upon thepulp, and indeed appreciably short of that point where the customarybleachingis obtained. This is done preferably by limiting the quantityof oxidizing agent to about the small amount stated above, but mayalternatively be done by the addition when bleaching has gone farenough, of an anti-chlor, usually sodium bi-sulfite, in an amountsufiicient to react with the residual oxidizing agent, and hence toneutralize or overcome its further eflects upon the charge of pulp whichis in process.

As a matter of regular control in commercial operations, and for thefurther insurance of preferred results in the ultimate product, therequisite amount of bleach may be so calculated as to effect the partialbleaching only of the batch under treatment, as for example to carry thebatch to approximately two-thirds of the usual bleach, and this quantityonly may be used. When this is done the bleaching or oxidizing capacityof the bleach expends itself at the desired point without the variationsincident to checking the progress of an incomplete reaction, withanti-chlors and the like.

Her'etofore, the pul after being subjected to a complete c lorine oroxidizing bleachin v dition o an alkaline pulp, comprising neubleach fora suitable length of time to. pro duce the desired color, was. eitherdrained and washed, or treated with an anti-chlor or reducing agentwhereby to prevent the further action of the bleaching agent, and thesoluble reagentsthen washed out, leaving the finished product. By thepresent invention it is found that a much higher quality of product isobtained by following such an incomplete oxidizing bleach as thatdisclosed above, with a reducing bleach as the final treatment.

To this end, therefore, a reducing bleach (containing SO as an essentialcomponent) is now added, such as a usual bi-sulfite cooking liquor,which is permitted to react for a short period (usually'a fewminutes'only is adequate) followed by separation of the pulp and washingthe pulp free from any substantial quantity of the reagents, with water.The bleaching action of this reagent und eir these conditions is markedand very rapi The resultant pulp is composed of long, relatively pureand strong alpha-cellulose fibres. They are essentially unaltered intheir physical properties but are separate and essentially free fromother components, associated materials and solids normally present inthe woods from'which it was prepared, and with very little degeneratedproducts of the cellulosic substance itself. It is moreovercharacterized by higher color, that is by a more brilliant white, thanany other pulp which is of equal strength (as manifested for example bypaper made therefrom) obtain able by oxidizing bleaching processes.

The process is unique in the preservation of a high yield of theoriginal alpha-cellulose, and the simultaneous production of a highlypurified pulp. The process further presents the great advantage ofpermitting the early exhaustion or early discontinuance of the oxidizingbleach and hence of eliminating its further and undesirable destructiveaction upon the pulp, which is not otherwise permissible where acommercially satisfactory white pulp is to be made.

I claim:

1. A method for the purification and bleaching of cellulosic materials,comprising treating the material with an acid-reducing agent, subjectingto an incomplete oxidizing bleach, and finally 2..A method for thebleachin of cellulosic materials, in the condition of an alkaline pulp,comprising neutralizing and bleaching the material with an acid reducingbleach, subjecting to an incomplete oxidizing bleach, and treating witha reducing bleaching agent.

3. A method for the purification and of cellulosic materials in theconpurification and tralizing the pulp with an acid reducing a ent,subjecting to an incomplete oxidizing b each, and treating with areducing bleaching reagent, followed by freeing the pulp from associatedreagent materials.

4. A method for the purification and bleaching of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing acid, subjecting to an incomplete oxidizingbleach, and treating with a reducing bleaching agent.-

5. A method for the purification and bleaching of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment,'neutralizing residualalkali with a reducing acid, sub ecting to an incomplete oxidizingbleach, and treating with a reducing bleaching reagent, said reducingacid and reducing bleach being the same reagent. I

6. A method for the purification and bleaching of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing acid, subjecting to an incomplete oxidizingbleach, and treating with a reducing bleaching reagent followed byfreeing the pulp from associated reagent materials.

7. A method for the purification and bleaching of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing sulfur dioxide bleach, subjecting to anincomplete oxidizing bleach, and treating with a reducing bleachingreagent.

8. A method for the purification and bleaching of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing solution containing sulfurous acid, subjecting toan incomplete oxidizing bleach, and treating with a reducing bleachingreagent.

9. A method for the purification and bleaching of cellulosic materials,reduced to a condition of a'pulpby a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing sulfite bleach, subjecting to an incompleteoxidizing bleach, and treating with a reducing bleaching reagent.

10. A method for the purification and bleachin of cellulosic materials,reduced to the condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment,"neutralizing residualalkali with a. reducing acid sulfite, subjecting to an incom leteoxidizing bleach, and treating with a re ucing bleaching reagent.

11. A method for the purification and bleachin of cellulosic materials,reduced to a condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing solution of calcium and magnesium bi-sulfite,subjecting to an incomplete oxidizing bleach, and treating with areducing bleaching reagent.

12. A method for the purification and bleaching of cellulosic materials,reduced to a condition of a pulp by a sulfite process, comprisingsubjecting the pulp to an alkaline treatment, neutralizing residualalkali with a reducing acid, subjecting to an oxidizing bleach,controlling the oxidizing bleach by adding the bleaching agent theretoin such amount as to be exhausted appreciably short of normal bleachingeffect, and thereafter treating with a reducing bleaching reagent.

Signed by me at South Brewer, Maine, this 30th day of January, 1926.

CHARLES A. BLODGETT.

